The present invention relates to a lubricating oil composition and, more particularly, to a lubricating oil composition for a continuously variable transmission of the metal belt type which exhibits a high friction coefficient between metals within a wide range of temperature from low temperatures to high temperatures.
Improvement in the fuel economy of automobiles is an important subject in recent years from the standpoint of the global environment. The continuously variable transmission of the metal belt type (hereinafter, abbreviated as a belt CVT) which is more efficient than conventional multistage automatic transmissions (AT) is being used in an increasingly greater number of automobiles. The belt CVT is used in automobiles having a wide range of displacement from 0.66 liters to 2.5 liters. The greater the displacement, the greater the required capacity of torque transfer of the metal belt, i.e., the higher the required friction coefficient between metals. On the other hand, there is the tendency that the use of the belt CVT increases worldwide and it is expected that the belt CVT will be used in a wide range of temperature from low temperatures to high temperatures. Therefore, it is desired that a lubricating oil for the belt CVT exhibits a high friction coefficient between metals in the wide range of temperature.
Investigations on the improvement of a lubricating oil for a belt CVT with respect to the friction coefficient between metals have heretofore been conducted only under conditions of high temperatures (Japanese Patent Application Laid-Open Nos. Heisei 11(1999)-92779, Heisei 11(1999)-293272 and 2000-1687) and no investigations are found on the improvement of the friction coefficient between metals at a low temperature such as several ten degrees Celsius below zero. Actually, when the friction coefficient between metals (xcexc) was measured using commercial lubricating oils for a belt CVT in accordance with the block-on-disk test, the highest values obtained were xcexc=0.138 at 100xc2x0 C. and xcexc=0.108 at xe2x88x9220xc2x0 C. Thus, it was found that the friction coefficient between metals decreased extremely at low temperatures.
The present invention has an object of overcoming the drawback of the above conventional technology and providing a high performance lubricating oil exhibiting a friction coefficient between metals in the use of a metal belt of xcexc=0.12 or higher at xe2x88x9220xc2x0 C.
As the result of intensive studies by the present inventors to overcome the above drawback, it was found that, although zinc dithiophosphate is used for improving the friction coefficient between metals in conventional lubricating oils for a belt CVT having a high friction coefficient between metals at 100xc2x0 C., the lubricating oil containing zinc dithiophosphate exhibited a great decrease in the friction coefficient between metals at low temperatures and the friction coefficient at the temperature of xe2x88x9220xc2x0 C. could not be increased to a value of 0.12 or higher and that it was necessary that a novel additive which could increase the friction coefficient between metals at low temperatures be added to overcome the above drawback.
As the result of further studies by the present inventors, it was found that the above object could be achieved by adding a sulfonate, phenate or salicylate of an alkaline earth metal in a specific amount or more. The present invention has been completed based on this knowledge.
The present invention provides a lubricating oil composition which comprises a lubricating base oil and a sulfonate, phenate or salicylate of an alkaline earth metal and has a friction coefficient between metals of 0.12 or higher at xe2x88x9220xc2x0 C. as measured in accordance with a block-on-disk test.